The invention relates to an endoprosthesis (joint prosthesis) frequently employed in orthopedic surgery. The prosthesis can be used for all types of endoprostheses, although the embodiment described hereinafter refers exclusively to a hip joint prosthesis.
The most varied indications may make it appear advisable to implant a substitute joint in a patient. Conventional endoprostheses, however, have only a service life of about five to eight years. The most frequent type of failure of such prostheses is their coming loose from the cement quiver or, for prostheses which have been inserted without cement, from the outer covering of the femur. The conventional endoprosthesis that has been inserted without cement has the drawback that the shaft extending into the bone (e.g. the femur) is insufficiently adapted to the shape of the marrow cavity intramedullary cavity due to the requirement for production in a certain number of sizes and the lack of a further curvature out of the single curvature plane (no distinction made between right and left). The resulting point contacts lead to locally very high transverse stresses and stress relief of the outer covering in the longitudinal directon and thus to resorptions in the upper region of the attachment of the prosthesis. The cemented endoprosthesis, although form-lockingly anchored in the cavity, constitutes, in its junction between prosthesis shaft and cement quiver, a component which is more resistant to bending by about one power of ten than the outer covering tube of the bone. The result is a reorientation of the flow of forces in the outer covering with large-area stress relief and local overstressing transverse to the orientation of the fibers.